It’s been clear that T-Mobile was planning to make with the details behind the next phase of its Uncarrier transformation today at the CES for a while now. Last month, we heard about this plan to offer subscribers to competing networks compensation for their early termination fees should they switch to T-Mobile, supposedly worth up to $350. Then early this morning we saw a leaked T-Mobile ad that seemed to confirm the news, while adding the note that it would only apply to users transition from the three big carriers, and was limited to five lines. This afternoon, T-Mobile has finally followed-through with its CES announcement, spelling-out all the details.

First, you will indeed need to be a current Verizon, AT&T, or Sprint user – sorry US Cellular folks. Then you bring your phone (or phones, if we’re talking about a family plan) to a T-Mobile store and trade-in your old handsets, where you’ll receive up to $300 a pop. T-Mobile will sign you up for new service (and while you can go with a Simple Choice plan with no contract, you still can’t take advantage of T-Mobile’s prepaid rates – this is postpaid only), sell you some new phones, and you can start using your T-Mobile gear straight away.

Then, when that final bill arrives from your old carrier, you send T-Mobile a copy and it will reimburse your ETF, up to $350 per line. That’s a potential payout of $650 per line, including the trade-in credit. We also see that five-line limit confirmed. Interested users can start taking T-Mobile up on this offer as of tomorrow.

So, pretty much just what we were expecting. Of course, by now AT&T has its own ETF buy-out plan, pulling the rug out from under T-Mobile a little, but it’s still a big step. With new avenues to competition opening up, will we see service rates start dropping? We can dream.